udev: keyboard-builtin: Add support for setting IBM trackpoint sensitivity

udev: keyboard-builtin: Add support for setting IBM trackpoint sensitivity

IBM / Lenovo trackpoints allow specifying a sensitivity setting through a
ps/2 command, which changes the range of the deltas sent when using the
trackpoint.

On some models with normal usage only deltas of 1 or 2 are send, resulting in
there only being 2 mouse cursor movement speeds, rather than the expected fluid
scale. Changing the sensitivity to a higher level than the bootup default fixes
this.

This commit adds support for setting a POINTINGSTICK_SENSITIVITY value
in hwdb to allow changing the sensitivity on boot through udev / hwdb.